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Metals fluorides (MFs) are potential conversion cathodes to replace commercial intercalation cathodes. However, the application of MFs is impeded by their poor electronic/ionic conductivity and severe decomposition of electrolyte. Here, a composite cathode of FeF 2 and polymer-derived carbon (FeF 2 @PDC) with excellent cycling performance is reported. The composite cathode is composed of nanorod-shaped FeF 2 embedded in PDC matrix with excellent mechanical strength and electronic/ionic conductivity. The FeF 2 @PDC enables a reversible capacity of 500 mAh g -1 with a record long cycle lifetime of 1900 cycles. Remarkably, the FeF 2 @PDC can be cycled at a record rate of 60 C with a reversible capacity of 107 mAh g -1 after 500 cycles. Advanced electron microscopy reveals that the in situ formation of stable Fe 3 O 4 layers on the surface of FeF 2 prevents the electrolyte decomposition and leaching of iron (Fe), thus enhancing the cyclability. The results provide a new understanding to FeF 2 electrochemistry, and a strategy to radically improve the electrochemical performance of FeF 2 cathode for lithium-ion battery applications., (© 2022 The Authors. Advanced Science published by Wiley-VCH GmbH.)

Replacing liquid electrolytes with solid polymer electrolytes (SPEs) is considered as a vital approach to developing sulfur (S)-based cathodes. However, the polysulfides shuttle and the growth of lithium (Li) dendrites are still the major challenges in polyethylene oxide (PEO)-based electrolyte. Here, an all-solid-state Li metal battery with flexible PEO-Li 10 Si 0.3 PS 6.7 Cl 1.8 (LSPSCl)-C-lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) composite cathode (FCC) and PEO-LSPSCl-LiTFSI composite electrolyte (S-CPE) is designed. The initial capacity of the Li|S-CPE|FCC battery is 414 mAh g -1 with 97.8% capacity retention after 100 cycles at 0.1 A g -1 . Moreover, the battery displays remarkable capacity retention of 80% after 500 cycles at 0.4 A g -1 . Cryo-transmission electron microscopy (Cryo-TEM) reveals rich large-sized Li 2 CO 3 particles at the Li/PEO interface blocking the Li + transport, but the layer with rich Li 2 O nanocrystals, amorphous LiF and Li 2 S at the Li/S-CPE interface suppresses the growth of lithium dendrite and stabilizes the interface. In situ optical microscopy demonstrates that the excellent cyclic stability of FCC is ascribed to the reversible shuttle of P-S-P species, resulting from the movement of ether backbone in PEO. This study provides strategies to mitigate the polysulfide shuttle effect and Li dendrite formation in designing high energy density solid-state Li-S-based batteries., (© 2022 Wiley-VCH GmbH.)

Conversion-type cathodes such as metal fluorides, especially FeF 2 and FeF 3 , are potential candidates to replace intercalation cathodes for the next generation of lithium ion batteries. However, the application of iron fluorides is impeded by their poor electronic conductivity, iron/fluorine dissolution, and unstable cathode electrolyte interfaces (CEIs). A facile route to fabricate a mechanical strong electrode with hierarchical electron pathways for FeF 2 nanoparticles is reported here. The FeF 2 /Li cell demonstrates remarkable cycle performances with a capacity of 300 mAh g -1 after a record long 4500 cycles at 1C. Meanwhile, a record stable high area capacity of over 6 mAh cm -2 is achieved. Furthermore, ultra-high rate capabilities at 20C and 6C for electrodes with low and high mass loading, respectively, are attained. Advanced electron microscopy reveals the formation of stable CEIs. The results demonstrate that the construction of viable electronic connections and favorable CEIs are the key to boost the electrochemical performances of FeF 2 cathode., (© 2022 Wiley-VCH GmbH.)

High interfacial resistance and uncontrollable lithium (Li) dendrite are major challenges in solid-state Li-metal batteries (SSLMBs), as they lead to premature short-circuiting and failure of SSLMBs. Here, we report the synthesis of a composite anode comprising a three-dimensional LiCu x nanowire network host infiltrated with Li (Li* anode) with low interfacial impedance and superior electrochemical performance. The Li* anode is fabricated by dissolving Cu foil into molten Li followed by solidification. The Li* anode exhibits good wettability with Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) and high mechanical strength, rendering low Li*/LLZTO interfacial impedance, homogeneous deposition of Li, and suppression of Li dendrites. Consequently, the Li* anode-based symmetric cells and full cells with LiNi 0.88 Co 0.1 Al 0.02 O 2 (NCA), LiFePO 4 (LFP), and FeF 2 cathodes deliver remarkable electrochemical performance. Specifically, the Li*/LLZTO/Li* symmetrical cell achieves a remarkably long cycle lifetime of 10 000 h with 0.1 mA·cm -2 ; the Li*/LLZTO/NCA full cell maintains capacity retention of 73.4% after 500 cycles at 0.5C; and all-solid-state Li*/LLZTO/FeF 2 full cell achieves a reversible capacity of 147 mAh·g -1 after 500 cycles at 100 mA·g -1 . This work demonstrates potential design tactics for an ultrastable Li*/garnet interface to enable high-performance SSLMBs.